Cretaceous, island-arc type, magmatic evolution resulted in the formation of the Srednogorie volcanic intrusive zone. The Chelopech mineral deposit is located within the Panagyurishte metallogenic district, a central part of the Srednogorie zone.

The Chelopech region consists of a Precambrian metamorphic basement consisting of gneisses, amphibolites, and metasediments overlain by Upper Cretaceous, volcanosedimentary sequences which include the Chelopech formation; the primary host to mineralisation.

The Chelopech Formation reaches thicknesses of up to 2,000 m and consists of Lower and Upper units.

The Lower Chelopech Unit is comprised of a basal sequence of siltstones and calcareous argillites with subordinate terrigenous sandstones and angular conglomerates. Upwards these sediments become intercalated and eventually superseded by volcanic sequences including andesites, andesitic agglomerates, andesitic lapilli and psammitic tuffs.

The Upper Chelopech Unit of Coniacian-Santonian age (Lower Senonian) comprises a complex of andesitic and dacitic lavas and tuffs with siliciclastic, volcaniclastic and argillaceous sediments intruded by sub-volcanic bodies of porphyritic andesites. The Upper Chelopech Formation passes up and laterally from mixed (terrigenous-volcanogenous) gritty sandstones, with volcanogenic exhalative iron-manganese oxide horizons, into volcanogenic talus breccias and agglomeratic tuffs of andesitic affinity.

Three successive mineralisation stages have been recognised at Chelopech, including an early Fe-S stage consisting mainly of disseminated and massive pyrite, a second Cu As-S stage which is the economic Cu and Au stage, and a late Pb-Zn stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.

The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. The mineralised zones vary from 40 to 200 m in length, are 20 to 130m thick, and can extend at least 390 m down plunge.

Production from underground is attained via Sublevel Longhole Open Stoping. The various ore bodies are developed at nominal 30 m vertical intervals and accessed by major declines in both, the Western and Central areas. Stopes are designed to be 20 m wide between the levels. The length of the stope depends on the geotechnical conditions, but can range between 20 and 60 m. The new trend of stope design is to keep a 20-30m length and 60m height, where geological and geotechnical conditions are suitable. This allows for improvement in ore handling and dust suppression during ore mucking as a result of shorter remote loading.

Once mined via an “end-slice” methodology, stopes are backfilled with “paste fill” produced from the mill tailings, to which cement is added and which is gravity fed underground via a system of borehole and pipes to the stopes being filled.

After commissioning, the integrated Chelopech operations are expected to produce a nominal 22,000 t of copper metal and up to 137,000 oz of gold bullion per annum from mining and processing 2 Mt of ore, by flotation, pressure oxidation (POX) and SX/EW for copper, and cyanidation of the residue for gold.

To overcome this significant hurdle, Chelopech is updating its flotation concentrator to improve efficiency and in anticipation of its feeding to the MPF, based on Pressure Oxidation (POX), in the future.

Discharge from the autoclave goes to the counter current decantation (CCD) circuit. Solid-liquid separation of the POX discharge allows subsequent treatment of the solution phase (containing the copper, zinc and other base metal values) from the solid phase (containing the gold values). The separation is to be achieved using a five-stage CCD circuit incorporating 13 m diameter high rate thickeners. Return water from the CIL is employed as wash water. The liquor from the CCD circuit reports to the PLS storage pond. The underflow from the final stage thickener is pumped to the CIL neutralization system.

The SX mixer settlers are configured for two stages of extraction and one stage of stripping. The aqueous phase exits the SX circuit as the raffinate stream and gravity feeds to the raffinate pond. The pond has two sections, a pre-settler with a floating boom for organic recovery and a surge pond.

In the Chelopech EW plant, advance electrolyte from copper SX will be fed to the copper EW circuit after removal of entrained organic. Solution temperature will be controlled to 45°C to achieve optimal conditions in electrolysis. This is achieved via heat exchange with the spent electrolyte followed by trim heating by direct steam injection.